1. Field of the Invention
The present invention relates to a zoom lens system and a camera incorporating it and is preferably applicable to a video camera and a digital still camera, etc.
2. Description of the Related Art
With the recent reduction in size and weight of a home video camera, etc., remarkable progress has been made in the miniaturization of image-taking zoom lens systems and energies are devoted in particular to shortening overall lengths of the zoom lens system, reduction of the diameter of a front lens element and simplification of a lens structures.
As one of means for achieving these objects, a so-called “rear focusing type zoom lens system” which moves lens units other than a 1st lens unit closest to the object side to achieve focusing is known.
The 1st lens unit of a rear focusing type zoom lens system generally has a smaller effective diameter than a zoom lens system which moves the 1st lens unit to achieve focusing and facilitates the reduction of the overall zoom lens system. It also facilitates short distance image-taking or extremely short distance image-taking in particular. Moreover, since small and light lens units are moved, it has features such that only a small driving force is required for the lens units and quick focusing is allowed, etc.
A conventional rear focusing type zoom lens system which is proposed in Japanese Patent Application Laid-Open No. H7(1995)-270684 and Japanese Patent Application Laid-Open No. H11(1999)-305124 comprises four lens units; a 1st lens unit with positive optical power, 2nd lens unit with negative optical power, 3rd lens unit with positive optical power and 4th lens unit with positive optical power arranged in that order from the object side, performs magnification varying by moving the 2nd lens unit and moves the 4th lens unit to correct aberrations to the image plane accompanying the magnification varying and achieve focusing.
In order to improve the ability to store a camera when not in use, it is generally effective to adopt collapsible lens units. However, for the above described zoom type zoom lens system in which the 2nd lens unit has most of the variable-magnification function, the 1st lens unit and 2nd lens unit are too sensitive relating to eccentricity to be suitable for the use as the collapsible type lens units.
In contrast to this, there is a known optical system which is proposed in Japanese Patent Application Laid-Open No. H10(1998)-62687 provided with a zoom lens system having a magnification-varying ratio of about three times, which comprises four lens units; a 1st lens unit with positive optical power, 2nd lens unit with negative optical power, 3rd lens unit with positive optical power and 4th lens unit with positive optical power arranged in that order from the object side, wherein the 1st lens unit consists of a single lens element, zooming is carried out by moving the 2nd, 3rd and 4th lens units and focusing is performed by moving the 4th lens unit to thereby simplify the optical system and provide an optical system which is also suitable for a collapsible structure.
There is also a zoom lens system which is proposed in Japanese Patent Application Laid-Open No. 2001-194586 having a magnification-varying ratio of about three times, which consists of four lens units; a 1st lens unit with positive optical power, 2nd lens unit with negative optical power, 3rd lens unit with positive optical power and 4th lens unit with positive optical power arranged in that order from the object side, wherein the 1st lens unit consists of a single lens element and zooming is carried out by moving the 1st, 2nd, 3rd and 4th lens units.
In order to reduce the size of the optical system, attempting to increase optical power of each lens unit of the optical system and at the same time reduce the number of lens elements will increase the thickness of the lenses, make the effect of shortening the zoom lens system insufficient and make it difficult to correct various kinds of aberrations.
Furthermore, when the zoom lens system is not used, attempting to collapse each lens unit for mounting will inevitably result in more errors due to inclinations, etc., of the lens elements and the lens units for reasons related to their mechanical structure. For this reason, the large sensitivity of lens elements and lens units result in deterioration of optical performance and jitter of images during zooming, and therefore it is preferable to reduce the sensitivity of the lens elements and lens units to a smallest possible level.
Attempting to perform zooming by moving only the 2nd lens unit and 4th lens unit of the four-unit zoom lens system consisting of positive, negative, positive and positive refractive power lens elements will cause the 2nd lens unit to be responsible for most of the magnification-varying operation, which inevitably requires the optical power of the 1st lens unit and 2nd lens unit to be increased.
In contrast to this, the optical system disclosed in Japanese Patent Application Laid-Open No. H10(1998)-62687 includes the 1st lens unit and 2nd lens unit with relatively small sensitivity, and is therefore suitable for a collapsible structure. However, since the 1st lens unit is fixed during zooming, shortening of the overall length of the zoom lens system at the wide-angle end and reduction in the diameter of the front lens element are not necessarily sufficient.
On the other hand, the optical system disclosed in Japanese Patent Application Laid-Open No. 2001-194586 realizes a small system with a large diameter and high performance by moving the 1st lens unit during zooming. However, since the amount of movement of the 1st lens unit from the wide-angle end to the telephoto end during zooming is small, the entrance pupil cannot be made short enough at the wide-angle end and the diameter of the front lens element (the lens element closest to the object) cannot necessarily be reduced enough, either.